Hybrid line with cable and overhead line is widely used in urban power grid, but it puts forward higher requirements for relay protection and reclosing configuration compared to single overhead line. Because faults of the overhead line are mostly transient faults, insulation could be restored, while faults of the cable are mostly permanent faults, launching of reclosing would lead to a second impact after the failure. In order to ensure cable safety, reclosing in a significant number of hybrid lines with cable and overhead line is not launched, thus transient faults will also lead to long-term power outage of the whole line, which reduces reliability of power supply.
In order to improve reliability and safety of power supply in the hybrid line, reclosing of the hybrid line should possess a self-adaption function, i.e.: reclosing is automatically launched when the fault point is located in the overhead line segment, reclosing is shut down when the cable line fails, and section locating of the hybrid line is the basis of self-adaptive reclosing.
A plurality of research units at home and abroad have carried out the relevant research work in the field of section locating of the hybrid line, and mainly proposed three schemes concerning: travelling wave fault location, distance component as well as installing transformer and protecting device, and the principles are as follows:
(1) Section locating based on travelling wave fault location: travelling wave fault location devices are installed on both sides of the line, the fault point is located through double-end travelling wave fault location, and section locating is realized in conjunction with the line structure.
(2) Reclosing cooperation based on distance component: a protection component is provided between two distances for line protection, one of the distances is set as the cable segment from the bus to the demarcation point, the other one is set as the overhead line segment from the bus to the demarcation point, and fault section is identified through cooperation of different impedance segments.
(3) Section locating by installing transformer and protecting device: a current transformer and an optical-fiber current differential protection are installed at the demarcation point between the cable and the overhead line, when a fault occurs, the differential protection can precisely judge the fault section, thus realizing requirements for shut-down of reclosing for the cable segment fault and switch-on of reclosing for the overhead line segment fault.
At present, the section locating methods of hybrid lines described above are not widely used due to restriction of techniques and costs. The section locating technique of hybrid lines based on a distance component is easy to realize, but it is technically difficult to overcome errors resulting from the transformer and line parameters, and it is greatly influenced by transition resistance. The section locating method of hybrid lines based on travelling wave principles and installed with transformer has the advantage that theoretical precision of algorithms is high, but it needs cooperation of devices at two ends and support of GPS and optical-fiber communication, in one aspect the cost is high, in the other aspect substantially there is no on-site implementation condition for a large number of hybrid lines of medium and low voltage classes. Meanwhile, the section locating method relying on communication cooperation is also difficult to meet time requirement for cooperating with reclosing.
Therefore, the invention provides a fault point locating method of hybrid lines based on analysis of comprehensive characteristics of single-end electric quantity and transient travelling waves.